【DFS】Combinations

Given two integers n and k, return all possible combinations of k numbers out of 1 ... n.

For example,
If n = 4 and k = 2, a solution is:

[
  [2,4],
  [3,4],
  [2,3],
  [1,2],
  [1,3],
  [1,4],
]

解法一：递归DFS，在Subsets I的基础上限制个数

public class Solution {
    public ArrayList<ArrayList<Integer>> res = new ArrayList<ArrayList<Integer>>();
    
    public void dfs(ArrayList<Integer> l, int n, int k, int pos){
        if(l.size() > k) return; 
        if(l.size() == k){
            res.add(new ArrayList(l));
            return ;
        }
        
        for(int i=pos; i<=n; i++){
            l.add(i);
            dfs(l, n, k, i+1);
            l.remove(l.size() - 1);
        }
    }
    
    public ArrayList<ArrayList<Integer>> combine(int n, int k) {
        ArrayList l = new ArrayList<Integer>();
        dfs(l, n, k, 1);
        return res;
    }
}